Guitarist Cheetah Chrome, back in his notorious days as guitarist for the Dead Boys (left) and Rupert Grint, who studied up to play Chrome in "CBGB" by watching videos of and reading about the Dead Boys.

The rule with rock ’n’ roll movies is to tell the legend. But, as Cheetah Chrome of 1970s punk band the Dead Boys tells The Post, the myth of how the band added their own secret ingredient into the house chili at CBGB is one that simply defies logic.

“We didn’t j – – k off into it,” says the guitarist, 58, even though that action is strongly hinted at in “CBGB,” a new biopic of the infamous New York club. “Why would we do that? We had no money, so that chili was our sustenance!”

Today, the former New Yorker lives in Nashville, Tenn.Anna O'Connor

“CBGB,” which opens Friday, certainly captures the spirit of the era. But some of the casting decisions initially raised eyebrows — like the news that Chrome would be portrayed by Rupert Grint. After all, the actor is known for playing Hogwarts alumni Ron Weasley in the “Harry Potter” series, while the Dead Boys’ notoriety includes trashing hotel rooms and abusing hard drugs. At one point, the Dead Boys had drunk so much at CBGB, their bar tab had run up to about $10,000 by Chrome’s estimate. (Also in the movie: Malin Akerman as Blondie, Mickey Sumner as Patti Smith and Justin Bartha as the Dead Boys’ self-mutilating frontman, Stiv Bators.)

Chrome says Grint nailed the role. “When I met him, he was prepared — he had watched videos and read up,” the musician says. “He was spot-on. When I passed him on set, it was like passing myself!”

Grint admits to The Post he was intimidated. “I was surprised Cheetah was still alive,” he says, laughing. “I went through a punk phase when I was young and listened to the Sex Pistols. But the Dead Boys make the Sex Pistols look like One Direction!”

One person not included in the movie is John Belushi. The comic was a big punk fan and CBGB regular. In 1978, he even sat in on drums with the Dead Boys at the benefit gig for drummer Johnny Blitz, who had been stabbed in an altercation.

“John wasn’t as crazy as everyone says,” Chrome says. “[Once,] he had me against the bathroom wall in CBs, telling me he was gonna kick my ass because he caught me doing heroin. Two weeks later, he was dead of the same thing. I sometimes wonder if I was doing something that made him curious.”